1. Field of the Art
The present relates to a cryopump and control method thereof, and in particular to the same in which an optional operation conditions can be provided and the regeneration and maintenance of the cryopump can be optimized.
More particularly, the invention relates to a cryopump and control method thereof in which stable operation can be maintained even if a sudden load change occurs in the cryopump, maintenance and checking can be performed at an appropriate time, complete regeneration of the cryopump can be performed in a short period, and the temperature of a cryopanel can be controlled without using a heater.
2. Prior Art
Up until now, to operate a cryopump under good operational condition, various cryopumps have been proposed, such as described in Published Unexamined Japanese Patent Application No. 152353/1991 (H3-152353), Published Unexamined Japanese Patent Application No. 237275/1991 (H3-237275) and the like.
In the cryopump described in the Application No. 152353/1991, a driving current is supplied to a driving motor or an expander motor of an expander, and when a value of the driving current detected varies unusually, a correction signal related to the unusual variation of the driving current is output to an invertor, and a rotational speed in the driving motor is lowered. Therefore, the driving motor is driven stably, and a synchronism loss phenomenon thereof can be avoided.
In the cryopump described in Application No. 237275/1991, an inverter means of a driving motor or an expander motor in a refrigerator is controlled based on a temperature in a cooling stage or pressure in a vacuum chamber to be evacuated, and, thereby, the rotational speed in the driving motor is determined.
The operational principal of a cryopump is based on the adsorption and the condensation of gas, and operational characteristics (or operational performance) of the cryopump is essentially affected by the adsorption and condensation of gas in the past, i.e. by the operational history of the cryopump.
However, in the above prior art, the rotational speed of the expander motor is controlled based on only the operational conditions at that time without considering the past operational history of the cryopump. In other words, the control of the rotational speed of the cryopump is limited to only a real time control.
Therefore, the following problems arise.
(1) FIG. 4 shows a rotational speed of an expander motor with respect to an operation elapsed time of a cryopump operated under conventional real time control.
As shown in FIG. 4, the expander motor is initially operated at the highest speed to perform rapid cooling of the cryopump, and is then operated at a lower stable rotational speed after cooling of the cryopump. However, in the case where a sudden load change occurs in the cryopump (for example, in the case where a sputtering operation is performed in a vacuum chamber to which the cryopump is attached) as shown in FIG. 4 by arrows "a", to maintain a temperature or a pressure in the vacuum chamber at a constant level, the rotational speed in the expander motor rapidly changes each time sputtering is performed. Therefore, an excess load is applied to the expander motor. In addition, a material constituting a seal of an expander which is driven by the expander motor is adversely affected, and is rapidly worn. Therefore, the working of the expander motor is shortened.
FIG. 5 shows pressure variation in a vacuum chamber. As shown in FIG. 5, though a pressure in the vacuum chamber is normally set to be 10.sup.-9 torr, the pressure is temporarily increased to 2.times.10.sup.-3 torr when sputtering is performed. Thus, at this time, as shown in FIG. 4 by arrows "a", the rotational speed of the expander motor is rapidly increased.
(2) The cryopump is utilized as a vacuum pump, and argon, water and hydrogen are adsorbed and accumulated on a cryopanel of the cryopump. Therefore, it is required to periodically remove the accumulated substance. In other words, a regeneration of the cryopump is required. Up until now, however, suitable time for maintenance work and checking for example, the regeneration of the cryopump cannot be properly determined. Therefore, the operational performance of the cryopump may suddenly deteriorate during operation, and the operation of the cryopump may be frequently stopped.
When deterioration of the operational performance of the cryopump suddenly occurs in a vacuum system such as a semiconductor manufacturing apparatus or the like, considerable damage can result.
(3) Deterioration of a cryopump with over time cannot be predicted or diagnosed, and, therefore, problems which may be caused by the deterioration of the cryopump with over time cannot be prevented.
(4) The reasonable and planned maintenance and checking adapted to each of various types of deterioration of the cryopump with over time cannot be performed. Therefore, wasteful maintenance and checking resulting in increased costs are required.
(5) To maintain the operational performance of a cryopump, which involves maintaining the temperature or pressure at a constant value, the cryopump is forcibly operated, and there is a probability that irreversible damage will occur.
Next, in usual two-stage cryopumps, a first stage cryopanel is maintained at a temperature of 50-100 K. to condense mainly water, and a second stage cryopanel is maintained at a temperature of 20 K. or lower to condense argon (Ar) and nitrogen (N.sub.2) gases. Also, an activated charcoal layer or the like formed on the reverse side of the second stage cryopanel cryogenically adsorbs hydrogen (H.sub.2) gas which cannot be condensed at temperatures of 20 K. or so and, thereby, a chamber is placed under vacuum.
A cryopump is a storage type vacuum pump as described above, and hence requires regeneration (release of condensed or adsorbed gases from a cryopanel) after running for a certain period of time. Since the chamber cannot be evacuated during regeneration, operation of a sputtering system and an ion implantation must be suspended. To improve availability of the systems, the regenerative time should be reduced to be as short as possible.
PCT Application Domestic Announcement No. 509144/1993 discloses a conventional regenerative technique for cryopanel surfaces of a cryopump run by a helium refrigerator. According to the regenerative technique shown, at the time of regenerating a cryopump, substances condensed/adsorbed on the cryopanel surface of a cryopump are changed in phase to a liquid phase and/or a gas phase, and the substances in the liquid phase and/or gas phase are exhausted from the cryopump for removal therefrom.
The prior art described above has an advantage of rapid regeneration because partial regeneration is employed, i.e. substances condensed/adsorbed on the second stage cryopanel surface of a cryopump are changed in phase to a liquid phase and/or a gas phase, and the substances in the liquid phase and/or gas phase are exhausted from the cryopump for removal therefrom. The regenerative method of the prior art, however, involves the following disadvantages (1)-(3).
(1) Due to partial regeneration, an internal temperature of a pump casing is maintained, during regeneration, below a temperature of melting and evaporating of water condensed on a cryopanel located at a first stage, i.e. the first stage cryopanel is not regenerated. However, in order to regenerate gases condensed or adsorbed on the second stage cryopanel, the pump casing temperature must be raised above a triple point of the gases. This causes the temperature of the first stage cryopanel to rise above that at the time of running as a cryopump. As a result, water condensed on the first stage cryopanel surface is caused to sublimate. According to the prior art described above, however, since the pump casing is evacuated only to a vacuum of 10 Pa or so after the regeneration, the sublimated water adsorbs, in the form of vapor (H.sub.2 O), on an activated charcoal layer provided on the back side of the second stage cryopanel. This causes the volume of adsorption of H.sub.2 to decrease in the next exhausting operation.
(2) Since substances are exhausted in the liquid phase and/or gas phase, two waste systems, i.e. gas and liquid systems are installed to treat the exhausted substances. As a result, the equipment becomes complex with a resultant increase in costs. Also, the process of treating the exhausted substances becomes complex.
(3) There has been a limit to effect a reduction in regenerative time. That is, only the time of partial regeneration has been able to be reduced, but an entire regenerative time is not reduced.
Further, as stated above in a conventional cryopump, a working gas, typically, a helium gas, at room temperature and high pressure supplied from a compressor unit is adiabatically expanded by an expander driven by an expander motor so as to generate cryogenic temperatures. The first stage cryopanel is cooled to a temperature of from 50 to 100 K. by a cooling gas generated in a first stage expanding portion of a helium refrigerator. On the other hand, the second stage cryopanel is cooled to a temperature of from 10 to 20 K. by a cooling gas generated in a second stage expanding portion of the helium refrigerator.
In such a cryopump, water or the like is condensed on the first stage cryopanel which is cooled to a temperature of from 50 to 100 K., while a nitrogen (N.sub.2) gas, an argon (Ar) gas or the like are condensed on the second stage cryopanel which is cooled to a temperature of from 10 to 20 K. A hydrogen (H.sub.2) gas or the like, which cannot be condensed on the second stage cryopanel cooled to 10 K., is further cryogenically adsorbed onto an activated charcoal layer provided on the back surface of the second stage cryopanel. The cryopump is thus used for forming a high vacuum in a vacuum chamber for a sputtering system or an ion implantation.
A conventional cold trap generally has a single stage cryopanel and in which a working gas, typically, a helium gas, at room temperature and high pressure is supplied from a compressor unit to be expanded adiabatically by an expander driven by an expander motor so as to generate cryogenic temperatures. The cryopanel is cooled to a temperature of from 80 to 130 K. by a cooling gas generated in a single stage expanding portion of a helium refrigerator.
A cold trap is typically placed upstream of a turbo molecular pump and has the capability to improve the pumping speed of water, which otherwise hampers the discharge performance of the turbo molecular pump. The cold trap permits water or the like to be condensed on the cryopanel cooled to a temperature of from 80 to 130 K. so that it can be used to form a high vacuum in a vacuum chamber in a sputtering system or an ion implantation.
In these apparatuses employing a cryopump and a cold trap, for example, in sputtering apparatuses, it is very important to maintain the uniformity of the sputter film, which requires that the pumping speed of the cryopump and that of the cold trap be kept constant. This further necessitates that the surface(s) of the first and/or the second stage cryopanels of the cryopump and the surface of the cryopanel of the cold trap be maintained at predetermined temperatures.
Further, since the cryopump and the cold trap discharge gases from a vacuum chamber while storing them therein (storage type), it is necessary to regenerate the gases (outgassing) after each discharge operation for a certain period of time. In the regenerating process, outgassing is performed after a gas is discharged and stored. It is thus necessary to maintain the cryopanels of the cryopump at approximately room temperature when it is desired that a gas condensed or adsorbed onto the surfaces of both the first and second stage panels be completely regenerated (complete or full regeneration), and when it is desired that a gas on only the second stage cryopanel be regenerated (partial regeneration), it is necessary to maintain the cryopump at a temperature of from 120 to 150 K. On the other hand, since the gas on the cryopanel of the cold trap is regenerated while the turbo molecular pump is driven, it is necessary that the cryopanel of the cold trap be maintained at a temperature of from -10.degree. to -30.degree. C. since water is required to be sublimed to perform outgassing.
In the conventional regenerate method, whichever method is employed for performing regeneration, a heater is used for maintaining the cryopanels of both the cryopump and the cold trap at constant temperatures. However, it is troublesome and costly to build a heater, and to arrange a circuit for supplying a current to the heater in a small casing of a cryopump or a cold trap. Additionally, if a heater is provided for a cryopump and a cold trap accommodated in a casing which is transformed in a high vacuum state, it may generate a gas, which may further produce an adverse influence on the vacuum processing side. Further, the temperature of the entire cryopanel cannot be uniformly adjusted with the heater, which also adversely influences the pumping speed and performance. Also, a sufficient regenerating operation cannot be achieved, and such localized heating may give rise to problems.
Therefore, an object of the present invention is to avoid the drawbacks of such a conventional cryopump, and provide a cryopump in which a sudden load change of an expander motor can be avoided, an operation at an optimized condition can be performed, and a suitable time for maintenance and checking required e.g. for the regeneration can be predicted.
Further object of the present invention is to provide a regenerative method and apparatus for a cryopump capable of regenerating cryopanels in a short period of time.
A still further object of the present invention is to provide a cryopump and a cold trap which can maintain the surfaces of the cryopanels at predetermined temperatures without requiring a heater.